Efficient charge separation after photon absorption by a semiconductor (SC) is one of the main challenges in the efficient harvesting of light energy. Improved electron-hole separation and inhibition of the photo-generated carriers (i.e. electrons or holes) are essential in improving the overall efficiency for interfacial charge transfer. In the last years several factors have been identified, which can have an important role in the separation and inhibition processes, like e.g. changes in phase composition, surface area, pore size distribution, particle morphology, particle aggregation, particle size distribution, bulk and surface defects, and impurities. Although some of these factors can be controlled to improve the performances, there is no general agreement of how some of the factors behave. For example, it is well established that crystallinity and purity are very important because they diminish the amount of defects, which can act as recombination centers. Therefore, efforts to produce highly crystalline and very pure materials to avoid defects and impurities, which increase the electron-hole recombination, have been made. But, the increase of surface area is also very important because it results in a higher density of active sites available for surface reactions (enhanced photocatalysis) as well as a higher interfacial charge-carrier transfer rate (better photovoltaic performances). It is also assumed that the recombination probability should decrease by decreasing the particle size, because the distance to the surface that carriers (formed in the interior of the SC) have to migrate to reach the active sites becomes short. Therefore, highly crystalline small particles, i.e. nanoparticles, which can increase the surface area, have been proposed (for a review of the role of nanoparticles in photocatalysis see, Journal of Nanoparticle Research, 1999, 1, 439-458). However, the rapid surface hole-electron recombination in nanoparticles makes that the photocatalytic and photovoltaic properties are lower than their larger counterpart.
Another important interfacial problem, mainly related with photovoltaic applications, is due to the poor interaction at the solid/solid junction, i.e. the interface between the semiconductor and the metals used for driving the current, reducing the efficiency.
The necessity of a good Ohmic contact at the semiconductor-metal interface as carrier collection is very inefficient when this interface is non-Ohmic, because such non-Ohmic contacts lead to higher overpotentials to attain a given anodic current. A metal-semiconductor junction results in an Ohmic contact (i.e. a contact with voltage independent resistance) if the Schottky barrier height is zero or negative. In such case, the carriers are free to flow in or out of the semiconductor so that there is a minimal resistance across the contact. For many semiconductors there is no appropriate metal available to get an Ohmic contact and different approaches (using thin layers, high doping levels, etc.) have to be applied to overcome this problem.
Overcoming such difficulty requires the use of intermediate layers/coatings to improve such interaction, but in any case, the efficiency is largely reduced.
Examples of solid/solid junctions between the semiconductor and the metals (metal nanoparticles) it is shown, for example, in the following patent documents:
WO 2008/102351 discloses nanoparticles comprising at least one metal/metal alloy region and at least one semiconductor region of a semiconducting material formed by elements combination selected from Groups II-VI.
WO 2011/011064 describes a method for photocatalytic splitting of water using hybrid nanoparticles which comprise a metal core and a semiconductor shell, wherein the metal core is made of a noble metal, such as Au, Ag, Pt, Pd or noble metal alloy and the semiconductor shell is made of e.g. TiO2, ZnS, Nb2O5.
Therefore, new ways to improve the separation of photogenerated charge carriers should be found for increasing the efficiency in photovoltaics and photocatalysis.
In WO 2007/017550 it is disclosed that Atomic Quantum Clusters are formed by less than 500 metal atoms, nevertheless many of them, due to their quantum confinement and the consequent separation of the energy levels, may not have a metal but semiconductor or insulating character, depending on their size.